In a near field communication technology (for example, Body-Area-Networks (BAN)), which assumes about one meter of a communication range, there have been developed techniques to convert energy found around an LSI (Large Scale Integration), such as light, heat, and vibration, into a source power.
Those techniques, called energy harvesting, reduce an apparent power consumption of an LSI. Furthermore, it is possible to save user's manual labor of charging or replacing batteries without reduction in performance of an LSI.
With those techniques, energy of light, heat, or vibration is converted into a source power for an LSI. Accordingly, those techniques require components such as a solar cell, a thermocouple, and an acceleration sensor. It is difficult to mount those components on an LSI with a standard CMOS. Therefore, those components cause an increase of cost in various ways upon energy harvesting. For example, elimination of batteries with the near field communication technology can save labor of replacing batteries or charging but increases the manufacturing cost.
Energy that can be recovered as a power source by an LSI, such as light, is unlimitedly present outside of the LSI. The problem results from the fact that a power converter component has a difficulty in performing a construction process using an existing CMOS circuit.
Accordingly, the inventor believes that the aforementioned problems can be solved by recovering energy from a signal having an energy form that can be processed in a CMOS circuit. The energy form that can readily be processed in a CMOS circuit is the form of an electric signal. The energy of an electric signal can be collected as a power source through the energy of an electromagnetic wave (environmental electromagnetic wave or environmental radio wave) coming from the outside of an LSI.
In order to produce a power source that has no supply source from a device such as a battery and operates with only an environmental radio wave, it is necessary to know the characteristics of an environmental radio wave emitted from radio equipment other than a desired correspondent communication apparatus. First, a radio source (1) having such a high level that it can be solely used as a power source has characteristics that a frequency band or the like varies along with time depending upon the communication environment. (For example, a radio source emitted from radio equipment located close to the target (apparatus).) Furthermore, a radio source (2) having uniform distribution of noise as compared to the radio source (1) has characteristics that an arrived power is very weak. (For example, a radio source emitted from a plurality of radio equipments located away from the target (apparatus).)
In the technology that considers the characteristics of the radio source (1), a radio wave having such intensity that a communication correspondent can use the radio wave as a power source is emitted at a fixed frequency to an LSI so as to supply power to the LSI. This is called a radio frequency identification tag (e.g., RfID). A configuration diagram of the prior art regarding an LSI is illustrated in FIG. 1 (Non-Patent Literature 1). Electromagnetic wave energy is supplied from a reader/writer 101 via antennas 102 to a radio frequency identification tag 103 and is then converted from an alternate current into a direct current by a full-wave rectifier 104. Then the energy is stabilized into a quality that is suitable for a circuit power source and is increased or decreased in voltage by an internal voltage control 106 and a booster 107. Thereafter, the energy is supplied as a power source to internal circuits such as a current-mode demodulator 109 and a logic 110. The conversion efficiency from an alternate current to a direct current can be increased by optimizing the device size of a PMOS rectifier diode 201 and an NMOS rectifier diode 202 included in the full-wave rectifier, of which configuration is illustrated in FIG. 2, or by reducing a loss due to a parasitic capacitor prior to an input to the rectifier diode. In this example, it is necessary to prepare a device for supplying electric power, such as the reader/writer device. This device is required as a separate device apart from data communication.
Furthermore, in the technology that considers the characteristics of the radio source (2), there has been proposed a technique that uses wide-band, high-efficient antennas to convert weak radio waves distributed in a wide band into a form that can be used as an electric power (Non-Patent Literature 2). This technique is regarded as energy harvesting technology using an environmental radio wave as a power source. Nevertheless, an antenna that is too large to be mounted on an LSI is required to recover a weak environmental radio wave. Consequently, this technique cannot fundamentally suppress an increase of cost of an apparatus for energy harvesting.
Furthermore, Patent Literature 1 discloses a technique of receiving electromagnetic wave energy supplied from a reader/writer device via antennas to a radio frequency identification tag and generating a direct-current voltage. The resonance frequency of the antennas in Patent Literature 1 can be adjusted depending upon the apparatus and is tuned to an electromagnetic wave frequency from the reader/writer device so that a value of the direct-current voltage is maximized in the radio frequency identification tag.